The present invention relates to furnaces, and, in particular, to a system for preheating material before it enters the furnace, using the off-gas from the furnace as the heating source.
Heating and melting furnaces, and in particular electric arc furnaces, are used to heat or melt various materials. Electric arc furnaces pass an electric arc (current) through ferrous metallic, non-ferrous metallic, and non-metallic scrap or other feed stock in order to melt the scrap.
During this production process, large quantities of very hot particulate-laden gases are generated. These gases create a problem, in that they are very hot, abrasive, difficult to handle, and they cannot be vented directly into the environment, due to the particulate material in them. These gases are usually drawn out of the furnace through a hole in the furnace roof. Ambient air or oxygen is introduced into the gas stream to allow ignition of any combustible gases or particulate materials. The gases are cooled, usually by passing through water-cooled ductwork, and are filtered through a baghouse, scrubber, precipitator, or other device with the function of collecting the particulate matter prior to discharge of the gas into the atmosphere.
If these off-gases can be used to preheat material to be charged into the furnace, several problems can be solved at once. First, the amount of time and energy required to melt the materials in the furnace can be reduced, resulting in considerable cost savings in the production process. Second, since the off-gases are cooled as they are used in the preheat process, the extensive cooling systems can be eliminated or greatly reduced, saving considerable capital and operating expense. Third, some of the particulate matter becomes trapped in the scrap, thus reducing the waste material to be disposed from the dust collection device.
There are several systems in use or proposed which are intended to use the off-gases to preheat the material to be charged into the furnace, and there are problems with all the existing systems. Some systems for using the off-gas to preheat require a complete redesign of the production process and equipment. Some other systems are simply not compatible with normal furnace operations, because they do not permit good operating practices or for other reasons. Most of the existing systems are very maintenance-intensive and/or have high operating expenses. In many designs, only part of the gases pass through the material to be preheated, sharply reducing the efficiency of the system. In some designs, the preheater cannot be bypassed if maintenance problems occur, which is clearly an undesirable situation. Some systems require additional processing of materials, particularly scrap, prior to use, or are limited to certain specific types or sizes of scrap.
Some systems preheat the charging bucket containing the scrap each time the scrap is preheated. Because the bucket has a significant weight relative to the contents, much energy is wasted in heating the bucket. This heating of the bucket is also damaging to the bucket, increasing maintenance costs.
In many designs, a new load of material to be preheated cannot be prepared or placed in position until after the furnace begins the next production cycle. The preheater is therefore not "on-line" as soon as the furnace begins the melting process, thus losing the opportunity to use all the off-gas stream for preheating.
Because of the space required for the preheat system, it typically is not located in the immediate vicinity of the melting furnace. When the preheat station is located at some distance from the furnace, several problems result. First, the heated vessel must be moved by crane or other device through the operating area to the furnace, resulting in a loss of preheat temperature and the escape of irritating or toxic fumes into the air. Second, in order to transport the vessel to the furnace area in time to quickly charge the furnace when the furnace is shut off, the vessel must be removed from the preheat station prematurely, thus resulting in a loss of, efficiency in the preheat process. Third, this movement of the vessel to the furnace by crane or other means is a double handling of the charging material, compromising normal operations. Fourth, due to the distance between the furnace and the vessel, much of the energy in the off-gas is lost before the gases reach the material to be preheated, resulting in a loss of efficiency and in only partial combustion of contaminants, such as oil, grease, plastics, and fibers, which produces an undesirable gas emission. Fifth, because the system is not near the furnace, great lengths of water-cooled ductwork are required to transport the gases to the preheating station, wasting energy and creating a large capital and maintenance expense.
The material to be charged into the furnace is usually scrap material that includes contaminants such as oils, greases, plastics, fibers, and water or ice. In the prior art, when the material is charged into the furnace, the residual heat in the furnace causes a flash ignition of these various combustible contaminants in the scrap, and a huge plume of flame and dust particles rises to the roof of the shop building. In most cases, large overhead canopies must be located above the furnace to capture this plume, which is then carried to the baghouse or other dust collecting device through ductwork attached to the canopies. These large canopies capture not only the plume, but also a very large volume of air from outside the furnace that travels up with the plume. This requires that the baghouse or other system for cleaning the air be sized very large to handle this large volume of air. Even with these overhead canopies and large air-cleaning systems, considerable dust remains in the air inside the building, which is undesirable. In some installations, separate enclosures are built around the furnace to capture charging emissions. Large doors are opened to permit a crane to place the charging bucket into the enclosure. The doors are closed, charging occurs, and the doors are opened to permit removal of the charging bucket. Use of such enclosures also requires that large volumes of air be collected to capture the charging emissions, although the air volume is somewhat less than that required for an overhead canopy system. Some collection of fugitive emissions in the roof area is still required. Such furnace enclosures are expensive, are easily damaged (particularly the large doors), and considerably disrupt efficient charging and operating practices.